The deceivingly simple appearance of batteries masks their chemical complexity. A typical lithium-ion battery in a cell phone consists of trillions of particles. When a lithium-ion battery is charged or discharged lithium ions move from one electrode to another, filling and unfilling individual, variably-sized battery particles. The rates of these processes determine how much power a battery can deliver. Despite the technological innovations and widespread use of batteries, the mechanism behind charging and discharging particles remains largely a mystery, partly because it is difficult to visualize the motion of lithium ions for a significant number of battery particles at nanoscale resolution.

Building Better Batteries

A battery consists of many particles, and understanding how they work in synergy continues to be an important focus for this research team. Between the positive electrode (cathode) and negative electrode (anode), an electrolyte allows ions (in this case Li ions) to travel from cathode to anode during charging (and to return during discharge).

While at Sandia National Laboratories, Dr. Chueh began this research to make a better battery cathode. Now, as a faculty member in Stanford’s Materials Sciences & Engineering department, Chueh has expanded this research project to further explore battery chemistry, to understand in greater depth how charging and discharging happens in situ, while the battery charging is taking place. The STXM capabilities at ALS Beamline 5.3.2 and 11.0.2 allow researchers to not only map the particles’ charges in freeze frame, but also enable in situ tracking during the charge/discharge process.

State-of-charge mapping obtained via scanning transmission x-ray microscopy (left) and morphology obtained via transmission electron microscopy (right) of the same regions in the lithium iron phosphate composite electrode (A) 26 μm, (B) 18 μm, and (C) 6 μm from the Al current collector. Hue and brightness give the local state-of-charge and lithium iron phosphate thickness, respectively. Outlined in white are particles in which two phases coexist within the same LFP particle; all other particles are single phase, either lithium-rich (red) or lithium-poor (green).

Researchers from Sandia National Laboratories and Berkeley Lab used synchrotron-based scanning transmission x-ray microscopy (STXM) at ALS Beamlines 5.3.2 and 11.0.2 to probe the charging and discharging dynamics of lithium iron phosphate (LFP), a promising positive battery electrode. By tracking the movements of lithium ions, one can decipher the process that ultimately limits the rate of battery charge and discharge. Previous work focused on either the behavior of a single battery particle or the spatially averaged behavior of the entire battery electrode. STXM gives a microscopic “lithium map” with a resolution down to 10 nm for hundreds of battery particles at a time. This unique visualization of lithium distribution across multiple length scales has generated significant insights into lithium-ion battery charging and discharging.

Researchers first charged commercial-grade battery cells to 50% full in 30 minutes, mimicking real world conditions. Then, the battery cell was immediately disassembled and the liquid electrolyte removed to lock in the lithium distribution. Next the battery was sliced into thin pieces about 500 nm thick using an ultramicrotome. Finally, the samples were brought to the ALS, where Fe L-edge spectromicroscopy was performed to determine the nanoscale distribution of oxidation states. In the charged, or de-lithiated state, the material adopts an Fe3+PO4 composition, whereas in the discharged, or lithiated state, the cathode material adopts a LiFe2+PO4 composition. Therefore, the local Fe oxidation state can be used to directly determine the local lithium content.

STXM couldn’t quite resolve individual particles, especially those smaller than 100 nm; complementary transmission electron microscopy (TEM) was performed on the same regions to discern one particle from another.

Researchers analyzed and quantified the local state-of-charge of approximately 500 individual LFP particles over nearly the entire thickness of the porous electrode. Using the STXM lithium maps and the high-resolution TEM images, researchers found that LFP battery particles do not charge simultaneously. Instead, the locked-in lithium distribution indicates that only about 2% of the battery particles were actively undergoing charging. The rest of the particles were either already charged or were yet to be charged.

X-ray absorption line scans of select lithium iron phosphate particles. Particles 1 and 2 are single phase and the x-ray absorption spectra showed negligible variations within the particle. Particle 3 contains two phases, separated by a well-defined boundary. Reference spectra (heavy lines) of the limiting compositions are also shown for comparison.

This simple observation has a significant implication: The battery particles actively undergoing charging (2%) carry all of the electrical current. In other words, the local charging current is about 50 times higher than the overall charging current. The researchers propose that such behavior is due to the high nucleation barrier that limits the rate at which LFP becomes de-lithiated (charged).

These results confirm a mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating phase transformation. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation. Moving forward, the researchers will apply this powerful combination of techniques to study other battery chemistries.

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); Sandia Truman Fellowship in National Security Science and Engineering; and Michigan State University. Operation of the ALS is supported by DOE BES.